Most semiconductor fabrication techniques include one or more ion implantation process steps. In order to maintain high yields, the ion implantation must be accurately controlled. A number of different approaches have been developed for measuring the dosage level in a semiconductor sample. One particularly advantageous approach is based upon the measurement of a modulated reflectance signal which can be related to the dosage level.
A modulated reflectance signal is obtained by periodically exciting the semiconductor sample with a focused, intensity modulated, pump laser beam. A probe beam is then reflected off the surface of the sample within the periodically excited area. The reflected power of the probe beam is measured to yield the modulated reflectance signal. This measurement is then correlated with the dosage level.
The above described approach provides a high resolution, non-contact technique. This technique is described in greater detail in U.S. Pat. No. 4,636,088 issued Jan. 13, 1987 and U.S. Pat. No. 4,854,710 issued Aug. 8, 1989 both assigned to the same assignee as the subject invention. As described therein, the modulated reflectance signal is effected by the presence of both thermal and plasma waves in the semiconductor sample.
The techniques described in the latter patents have been incorporated in a device manufactured by Therma-Wave, Inc. of Fremont, Calif. and marketed under the trademark Therma-Probe. The Therma-Probe device is being widely used by the semiconductor industry to monitor all types of ion implant processes. Up until now, this device has been limited to monitoring dosage levels up to 10.sup.14 to 10.sup.15 ions/cm.sup.2. Some semiconductor manufacturers are now using higher doses, up to 10.sup.16 ions/cm.sup.2.
The difficulty in extending the sensitivity of the device to the highest dosage levels is due principally to the fact that the rise in the measured modulated reflectance signal, which is monotonic below 10.sup.14 ions/cm.sup.2 and can therefore be directly correlated with dosage level, becomes oscillatory at higher dosage levels. The reason for this change in behavior in the modulated reflected signal has been attributed to the fact that the high level of ion implantation transforms the crystalline material into amorphous silicon. Amorphous silicon is created when more than ten percent of the atoms in the lattice structure are displaced. Amorphous silicon is characterized by the fact that no x-ray diffraction patterns can be generated since the lattice has been so severely disturbed.
Typically, a layer of amorphous silicon is created which lies just below an upper layer of less damaged silicon. The reason there is a layer of damaged (but still crystalline) silicon above the amorphous silicon layer depends upon the type of ions which are being implanted. For example, implantation using the arsenic ion requires relatively high energies. Some of this implantation energy is transferred to the material near the upper surface of the sample creating a dynamic annealing effect which recreates the crystalline structure. A different scenario occurs in boron implantation. More specifically, the boron ions are implanted with very high speed and literally pass through the upper layer before they have slowed down enough to interact with the lattice in an amount sufficient to create amorphous silicon. In either case, the result of high dosage implantation is to create an upper layer of damaged, crystalline silicon and an intermediate layer of amorphous silicon below.
The significance of the presence of the amorphous silicon layer is that this layer acts as a boundary which can reflect optical radiation. This reflection gives rise to interference effects which, in turn, effect the response of the modulated reflectance signal. This theoretical explanation was first described by the applicant herein in "Modulated Interference Effects and Thermal Wave Monitoring of High Dose Implantation in Semiconductors," Review of Progress in Quantitative Non-destructive Evaluation, Vol. 8B, 1989.
The latter article also describes how the thickness of the amorphous silicon layer can be related to the level of ion implantation. More specifically, as the level of ion implantation increases to high dosages, the thickness of the amorphous silicon layer will increase. The latter article also includes calculations indicating how both the modulated and non-modulated reflectance signals of a probe beam will behave as the thickness of the amorphous silicon layer increases. [See also "Modulated Optical Reflectance Measurements on Amorphous Silicon Layers and Detection of Residual Defects", S. Wurm, P. Alpern, D. Savignac, and R. Kakoschke, Appl. Phys. A 47, 147-155 (1988).]
The effects of the presence of an amorphous silicon layer on surface reflectivity was further discussed by the applicant in "High Resolution Thermal Wave Measurements and Imaging of Defects and Damage on Electronic Materials", J. Opsal, which was first presented in August 1989 and will appear in Photoacoustics and Photothermal Phenomena VI (Springer-Verlag 1990). The latter paper discusses how the reflectivity of a sample which has been implanted with a high level of ions could be analyzed using a general mathematical model of a multi-layer system. This approach provides a mathematical framework for relating the reflectivity of a sample to the thickness of the amorphous silicon layer. As noted above, the thickness of the amorphous silicon layer is related to the dosage level. Therefore, if one can determine the thickness of the amorphous silicon layer, the dosage level in the sample can be evaluated.
Unfortunately, simply setting forth a general mathematical model which defines the reflectivity response of a layered sample does not alone permit the calculation of the thickness of the amorphous silicon layer based on the measurement of the modulated reflectivity signal. More particularly, a typical implanted semiconductor sample consists of a number of layers each having an unknown thickness. As noted above, a typical sample includes an upper damaged crystalline layer and a lower layer of amorphous silicon. In addition, there is typically an upper mask layer formed from an oxide material. The thickness of all three layers is unknown, and thus a single measurement of the modulated reflectance signal cannot be used to solve multiple equations in a mathematical model which includes unknown values for three layer thicknesses.